fix(pkg/dockerfile): fix FROM in Dockerfile parsing issues

[xunleii]

This pull request improves the robustness of Dockerfile parsing in the pkg/dockerfile package by updating regular expressions to handle more Dockerfile syntax variations and adding new test cases to ensure correct parsing. The changes focus on better handling of whitespace, comments, and build stage aliases in FROM, LABEL, and ARG instructions.

Parsing improvements:

• Updated the regular expressions in dockerfile.go to allow for optional whitespace and to better capture image references, tags, and digests in FROM instructions, as well as to handle LABEL and ARG instructions with more flexible spacing.

Test coverage enhancements:

• Added new test cases in dockerfile_test.go to cover Dockerfiles with build stage aliases (AS keyword, including uppercase), inline comments in FROM instructions, and other edge cases.
• Added fixture Dockerfiles: simple-AS.dockerfile for testing AS build stage parsing [1] and simple-comment.dockerfile for testing parsing of FROM instructions with comments. [2]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.40%. Comparing base (8b82fe4) to head (7782653).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #930   +/-   ##
=======================================
  Coverage   55.40%   55.40%           
=======================================
  Files          55       55           
  Lines        3485     3485           
=======================================
  Hits         1931     1931           
  Misses       1430     1430           
  Partials      124      124           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

Improves pkg/dockerfile parsing robustness for Dockerfile instructions (primarily FROM) by relaxing regex matching to support more real-world syntax like build stage aliases and inline comments, and adds fixtures/tests to validate these cases.

Changes:

• Updated regexes in pkg/dockerfile/dockerfile.go to better handle whitespace and trailing content (e.g., AS aliases, comments).
• Added new Dockerfile fixtures covering FROM ... AS ... and FROM ... # comment.
• Expanded TestParseDockerfile with new cases for aliases and comments.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
pkg/dockerfile/dockerfile.go	Updates regexes used to parse FROM, LABEL, and ARG lines.
pkg/dockerfile/dockerfile_test.go	Adds test cases for AS (uppercase) and inline FROM comments; adds a multistage alias case.
test/fixtures/dockerfile/simple-AS.dockerfile	New fixture for FROM ... AS builder.
test/fixtures/dockerfile/simple-comment.dockerfile	New fixture for FROM ... # comment.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[antoinegelloz]

LGTM 👌